This invention relates generally to apparatus for inserting flexible members, such as tie-back anchors for slope stabilization or prefabricated vertical (PV) drains (sometimes referred to as wick or band drains) into the earth, and more particularly to an improved apparatus for inserting such members into dense or hard soil layers.
One well known technique for improving soft, saturated soil, such as wet clay, for example, is to drive into the soil a drainage element (PV drain) that penetrates deep into the soil with the top end of the drainage element maintained above the surface of the soil. The PV drain is formed of any suitable material which is water permeable, or perforated to be water permeable, so that the water in the soil can penetrate the walls of the drain and flow upwardly therein, to the surface of the soil as a result of water pressures in the soil beneath the surface. It is common practice in such situations to increase these inherent water pressures in the soil by placing a layer of earth on top of the wet soil so that the weight thereof will assist in forcing the water into and upwardly through the PV drains, where it can be readily disbursed.
The PV drain is generally elongated and flexible and it is carried into the ground by utilizing a rigid insertion tube or mandrel formed of suitable metal. This insertion tube, together with the drain contained therein, is driven downwardly into the earth to the desired depth and then the insertion tube is pulled out of the soil thereby leaving the PV drain. The drains are inserted at regular predetermined intervals in the earth, depending upon soil conditions and moisture content.
This rigid insertion tube or mandrel, which carries the elongated, flexible PV drain therein, is adapted for vertical movement within a mast. The insertion tube is forcibly driven into the earth, and then pulled out by any one of different known drive systems. For example, in Dutch Patent No. 7,707,303, there is disclosed a drive arrangement which uses a vibratory driver that engages the top portion of the insertion tube for driving the bottom end of the insertion tube into the earth. In Cortlever, U.S. Pat. No. 4,755,080, a combination of hydraulic cylinders and a cable drive that engages the insertion tube at the upper end thereof is utilized, and a somewhat similar hydraulic motor and chain drive is disclosed in Thorsell U.S. Pat. No. 3,891,186.
In general, most of these prior art arrangements engage and drive the insertion tube at its top end, requiring a relatively heavy mast and boom arrangement to support the insertion tube or mandrel and the drive mechanism. This not only increases the weight of the apparatus, but also increases the cost of fabrication as well as maintenance.
It is also known to utilize vibratory means in combination with cable or chain drives. These rigs are commonly referred to as vibro/static machines. In these machines a vibrator is mounted to the top of the mandrel to impart vertical vibration to the mandrel. Elastomers placed between the mandrel and the drive (chain, cables etc.) isolate the vibrations from the drive and mast. From a geotechnical standpoint, it is preferable to install wick drains without the use of vibration, since such vibration can remold the soil in close proximity with the mandrel, resulting in loss of strength and decreased permeability. Lower permeability of the soil in this region impedes the flow of water into the drain, requiring longer surcharge periods. However, vibration greatly enhances the ability of the apparatus to penetrate the ground, and it is often necessary to penetrate through dense or hard soil layers to reach an underlying soft soil layer. These layers are often so hard that it is not possible to penetrate them without the use of a vibratory system. The combination machines (vibro/static) are very useful in these cases, since the vibration can be turned on only during penetration through the hard layers. Further, vibrating the mandrel induces very high vibratory stresses, and fatigue of the mandrel material becomes a problem.
It is also known that the insertion tube can be driven into the earth utilizing a pair of friction rollers positioned just above the surface of the earth, these rollers being formed of a material that will frictionally engage the side walls of the insertion tube disposed therebetween with the frictional engagement between the rollers and the insertion tube, thus driving the insertion tube into the ground. This prior art friction roller arrangement overcomes the problem of engaging the insertion tube at its upper end, but suffers from a tendency of the friction rollers to slip when the mandrel or insertion tube is covered with wet, slippery soil material which adheres to the mandrel. The Morris Patent (U.S. Pat No. 5,213,449) overcomes this problem by utilizing a drive gear to positively engage a flange or fin which is attached to and coextends with the mandrel. This flange contains rack gear mesh openings spaced along its length, which the teeth of the drive gear engage. This arrangement is similar to a rack and pinion arrangement. These bottom-drive arrangements overcome the need to engage the mandrel at its top end, and require a mast sufficient to support the mandrel only. They cannot, however apply vibration to the mandrel for added penetrating ability.
Goughnour and Joiner (U.S. Pat. No. 5,658,091) disclose a vibro/static system whereby a vibratory driver is positioned at and attached to the upper end of the mandrel for imparting vibrations to assist in its penetration. A drive which includes a rotary drive gear, that engages a mandrel/fin, and a motor for driving the gear is mounted at the bottom of the mast as with the Morris Patent. A flexible torsion coupler between the motor and the drive gear isolates the motor and the mast from vibrations imparted to the mandrel by the vibrator. Although this system does not require static crowd engagement of the mandrel at its top end, the mast must be structurally sufficient to support the vibratory driver that travels to the top of the mast.
These same techniques are also utilized for inserting other flexible members into the earth, such as tie back anchors for slope stabilization.
The present invention discloses means to add vibratory capability to the bottom-drive apparatus of the friction roller type or of the type disclosed in the Morris or Goughnour/Joiner patents, wherein the vibratory driver is mounted to, and remains at the lower end of the mast. This permits application of vibrations to the mandrel either intermittently or constantly as required, but does not require the heavy mast structure to support a vibratory driver that travels to the top end of the mast.